Domain authority is the measure of strength and popularity of your domain in search engines. It is measured by Moz and your domain is ranked between 0 to100 depending on the popularity and trust. DA can be improved by good content update and also by building backlinks from other relevant domains.

The best candidates here are going to be people with link building based on whether you're going for general Domain Authority link building.Domain authority is good, but relevant links are even better for rankings, ... an increase in DA as a metric of a successful link building campaign.

Domain authority is an seo factor calculated by Moz and depends on more than 40 different ranking factors. However, DA is not an actual SEO ranking factor used by google. While considering DA you should also look at the DA of your competitors and also your industry, and aim to be better than them. There is not fixed DA threshold about a good site. It depends on the DA of your industry & your competitors. It's also important to pay attention to page authority as well as user experience.